Iron-based particles have long been used as a base material in the manufacture of structural components by powder metallurgical methods. The iron-based particles are first molded in a die under high pressures in order to produce the desired shape. After the molding step, the structural component usually undergoes a sintering step to impart the necessary strength to the component.
Magnetic core components have also been manufactured by such powder metallurgical methods, but the iron-based particles used in these methods are generally coated with a circumferential layer of insulating material. These compacted components generally are not sintered because that heating process would destroy the insulating material.
Two important characteristics of an iron core component are its magnetic permeability and core loss characteristics. The magnetic permeability of a material is an indication of its ability to become magnetized, or its ability to carry a magnetic flux. Permeability is defined as the ratio of the induced magnetic flux to the magnetizing force or field intensity. When a magnetic material is exposed to a rapidly varying field, the total energy of the core is reduced by the occurrence of hysteresis losses and/or eddy current losses. The hysteresis loss is brought about by the necessary expenditure of energy to overcome the retained magnetic forces within the iron core component. The eddy current loss is brought about by the production of electric currents in the iron core component due to the changing flux caused by alternating current conditions.
Research in the powder metallurgical manufacture of magnetic core components using coated iron-based powders has been directed to the development of iron powder compositions that enhance certain physical and magnetic properties without detrimentally affecting other properties. Desired properties include a high permeability through an extended frequency range, high pressed strength, low core losses, and suitability for compression molding techniques.
When molding a core component for AC power applications, it is generally required that the iron particles have an electrically insulating coating to decrease core losses. The use of a plastic coating over the iron particles (see, for example, U.S. Pat. No. 5,198,137 to Rutz et al.) and the use of doubly-coated iron particles (see U.S. Pat. No. 4,601,765 to Soileau et al.) have been employed to insulate the iron particles and therefore reduce eddy current losses. It has also been shown that the insulating coating provided by a polymeric material can be achieved by bonding the polymeric material to the iron-based powder (see U.S. Pat. No. 5,225,459 to Oliver et al.).
The compaction of the powder metallurgical compositions is carried out within a die cavity that is subjected to extreme pressures. To avoid excessive wear on the die cavity, lubricants are commonly used during the compaction process. Lubricants can be generally classified into two groups: internal (dry) lubricants and external (spray) lubricants. The internal lubricants are admixed with the metal-based powder composition, and the external lubricants are sprayed onto the die cavity prior to compaction. Lubricants are used to reduce internal friction between particles during compaction, to permit easier ejection of the compact from the die cavity, to reduce die wear, and/or to allow more uniform compaction of the metal powder blend. Common lubricants include solids such as metallic stearates or synthetic waxes.
As will be recognized, most known internal lubricants reduce the green strength of the compact. It is believed that during compaction the internal lubricant is exuded between iron and/or alloying metal particles such that it fills the pore volume between the particles and interferes with particle-to-particle bonding. Indeed, some shapes cannot be pressed using known internal lubricants. Tall, thin-walled bushings, for example, require large amounts of internal lubricant to overcome die wall friction and reduce the required ejection force. Such levels of internal lubricant, however, typically reduce green strength to the point that the resulting compacts crumble upon ejection. Also, internal lubricants such as zinc stearate often adversely affect powder flow rate and apparent density, as well as green density of the compact, particularly at higher compaction pressures. Moreover, excessive amounts of internal lubricants can lead to compacts having poor dimensional integrity, and volatized lubricant can form soot on the heating elements of the sintering furnace. To avoid these problems, it is known to use an external spray lubricant rather than an internal lubricant. However, the use of external lubricants increases the compaction cycle time and leads to less uniform compaction.
Accordingly, there exists a need in the art for metallurgical powder compositions used for magnetic applications that can be readily compacted to strong green parts that are easily ejected from die cavities without the need for an external lubricant.